ABSTRACT The results of new detrital zircon analyses of 15 ( n = 1334) Sevier belt synorogenic (Jurassic–Eocene) conglomerates combined with U-Pb zircon ages from the literature ( n = 2638) support the structurally dynamic role of the western Paris thrust sheet as the dominant high-standing, out-of-sequence portion of the Sevier belt. This result requires modification of the traditional structural view of the thin-skinned Sevier fold-and-thrust belt having formed by west-to-east shortening over an ~100-m.y. period (ca. 150–50 Ma) with episodic thrust motions that become younger toward the craton (east), as constrained by numerous synorogenic deposits shed to the east from each thrust hanging wall. Sevier thrusting was preceded by deposition of the Jurassic Stump Formation, which has a maximum depositional age of 149 Ma and a unique detrital zircon and heavy mineral (garnet, magnetite) provenance. The oldest thrust, the Paris (Willard) thrust, eroded and deposited the Jurassic–Cretaceous Ephraim Conglomerate as a synorogenic fan devoid of quartzite clasts and with a detrital zircon provenance consistent with reworked sediment from the fold belt, but not from the hinterland or the Sierra Nevada arc of the orogenic system. All subsequent synorogenic deposits from the mid-Cretaceous Echo Conglomerate (Meade-Crawford thrust) to a variety of more easterly Eocene deposits (Sevier belt, Green River, Absaroka, and Bighorn basins) are rich in quartzite clasts. All the quartzite clasts were eroded from the Paris thrust hanging wall, which reached its peak orogenic height at ca. 95 Ma, 50 m.y. after first motion, and the Proterozoic Brigham Group remained a quartzite clast source for ~40 m.y. The detrital zircon signatures of these samples require additional sources of sediment, reworked from the hinterland and the Sierra Nevada and Idaho Batholith arcs, thus implying that long-distance sediment fairway(s) were active during the Mesozoic–early Cenozoic. Based on the same detrital zircon data, variable sources of sediment are inferred between each of the thrust sheets; however, within each thrust system, the source of sediment remained the same. The Teton Range was thrust up at ca. 50 Ma, long after the Sevier belt formed, and it was not a buttress to thin-skinned Sevier deformation. Rather, Teton–Gros Ventre–Wind River Laramide uplifts deformed the older Sevier belt with numerous back and out-of-sequence thrusts and synorogenic deposits, including the Darby thrust, which records the youngest displacement.

Abstract A synthesis of low-temperature thermochronologic results throughout the Laramide foreland illustrates that samples from wellbores in Laramide basins record either (1) detrital Laramide or older cooling ages in the upper ~1 km (0.62 mi) of the wellbore, with younger ages at greater depths as temperatures increase; or (2) Neogene cooling ages. Surface samples from Laramide ranges typically record either Laramide or older cooling ages. It is apparent that for any particular area the complexity of the cooling history, and hence the tectonic history interpreted from the cooling history, increases as the number of studies or the area covered by a study increases. Most Laramide ranges probably experienced a complex tectono-thermal evolution. Deriving a regional timing sequence for the evolution of the Laramide basins and ranges is still elusive, although a compilation of low-temperature thermochronology data from ranges in the Laramide foreland suggests a younging of the ranges to the south and southwest. Studies of subsurface samples from Laramide basins have, in some cases, been integrated with and used to constrain results from basin burial-history modeling. Current exploration for unconventional shale-oil or shale-gas plays in the Rocky Mountains has renewed interest in thermal and burial history modeling as an aid in evaluating thermal maturity and understanding petroleum systems.This paper suggests that low-temperature thermochronometers are underutilized tools that can provide additional constraints to burial-history modeling and source rock evaluation in the Rocky Mountain region.

Google Earth Pro imagery was used by graduate students for a course project to identify, describe, and interpret lineament patterns on two oil-producing anticlines in Wyoming, one in the northwest Wind River Basin and the other in the southern Bighorn Basin (Maverick Springs and Thermopolis anticlines, respectively). These anticlines lie on opposite sides of the east-west–trending Owl Creek arch, which is a sinistral, transpressive array of en echelon, basement-involved thrust blocks. Both anticlines are well-exposed and display extensive near-surface fracturing and faulting, making them ideal candidates for a study of fold-related lineament patterns. Google Earth Pro was used to map and measure the orientation of lineaments and faults in a digital format. The lineaments identified include a set parallel to dip (A–C), a set parallel to strike (B–C), and two sets oblique to strike. Lineament orientation data were analyzed using length-weighted rose diagrams, whereas fold geometry and plunge were evaluated using equal-area (lower hemisphere) stereonets. Although the study was limited in scope to a computer-based geometric analysis and did not include outcrop-based kinematic data, the lineament/fracture data derived from Google Earth mapping are nevertheless compatible with published studies that demonstrate regional NE-SW shortening along the western Owl Creek transpressive zone during the Laramide orogeny. Google Earth Pro proved to be a highly effective tool for gathering lineament orientation and spatial distribution data across these well-exposed anticlines.